Boron carbide has extremely high hardness and modulus of elasticity and is used as a lightweight, high performance high temperature material. Powder and fiber forms of boron carbide are known. However, fibers of boron carbide are expensive and have a large diameter. Monolithic (non-fiber) forms of boron carbide are used at temperatures of up to at least 2100° C. in inert environments. Monolithic boron carbide has been used to make hundreds of thousands of personal body armor plates and also finds use in military and private vehicles where ballistic protection and lowest weight are of high importance. Monolithic boron carbide has also been used in nuclear moderator applications due to its recognized neutron moderation properties. However, for use in nuclear applications, the monolithic boron carbide requires an additional material to encase the boron carbide, such as a stainless steel sheathing.
Boron carbide powder is formed by reacting carbon with boron trichloride (BCl3) in the presence of hydrogen at 800° C. according to the following reaction: C(s)+4 BCl3(g)+6H2(g)→B4C(s)+12 HCl(g). This chemistry-driven process involves gas-solid heterogeneous reactions and is used in making boron carbide powder that contains excess carbon. Boron carbide powder is then compacted with a sintering aid and sintered at a temperature greater than 2000° C. to form various articles, such as plates, tubes, nozzles. Boron carbide fibers are formed by a carbothermal reduction reaction of boron and carbon powders in slurries placed on carbon, or deposited as a coating using a chemical vapor deposition (CVD) process. The CVD process deposits boron on carbon fibers or boron carbide directly onto carbon fibers using boron halides or diborane with methane or another chemical carbon source. The CVD process produces relatively large diameter (4 mm to 5.6 mm) fibers. The articles produced that include these boron carbide fibers usually contain excess boron or carbon.
One method of forming boron carbide fibers includes forming a pyrolytic coating on carbon or graphite filaments. The pyrolytic coating is applied to the filaments at reduced pressure and at a temperature between 1300° C. and 2100° C. A source gas that includes a hydrocarbon and a halide of boron is decomposed on the filaments to form the pyrolytic coating.
Another method of forming boron carbide fibers includes heating boric oxide fibers in an ammonia atmosphere to a temperature of 350° C. to 600° C. to produce fibers that include boron, nitrogen, oxygen, and hydrogen. The ammonia-treated fibers are then heated in an amine atmosphere at a temperature of 600° C. to 1000° C. to produce fibers that include boron, carbon, nitrogen, oxygen, and hydrogen. The amine-treated fibers are then heated to a temperature of 2000° C.-2350° C. in an inert atmosphere, producing the boron carbide fibers.
Another method of forming boron carbide fibers includes using diboron trioxide (B2O3) powder. The powder is dispersed in a water slurry and cellulose fibers added to the water slurry. The B2O3 is dispersed in the cellulose matrix and carbonized to produce boron carbide fibers in which the B2O3 is dispersed in the cellulose matrix.
It would be desirable to produce continuous fibers of boron carbide exhibiting enhanced thermal, mechanical, and neutron adsorption properties for use in a variety of articles. Further, it would be desirable to produce continuous fibers of boron carbide of a very fine diameter. In addition, it would also be desirable to produce the continuous boron carbide fibers in an economical manner.